This invention relates to a method and apparatus for measuring coin diameter.
The invention will be described in the context of coin validators, but it is to be noted that the term "coin" is employed to mean any coin (whether valid or counterfeit), token, slug, washer, or other metallic object or item, and especially any metallic object or item which could be utilised by an individual in an attempt to operate a coin-operated device or system. A "valid coin" is considered to be an authentic coin, token, or the like, and especially an authentic coin of a monetary system or systems in which or with which a coin-operated device or system is intended to operate and of a denomination which such coin-operated device or system is intended selectively to receive and to treat as an item of value.
One known technique for measuring the diameter of a coin involves using an electromagnetic coil as part of an oscillator circuit so that the frequency of the oscillator output is dependent upon the inductance of the coil. A coin is caused to move past the coil and the changing frequency is measured. This is indicative of coin diameter, because the frequency shift is determined by the change of inductance, which is in turn dependent upon the area of overlap between the coil and the coin. For effective results, the coil should be large, and preferably larger than the largest-sized diameter coin to be measured. The frequency of the oscillator should be high so that the measurement is substantially unaffected by coin thickness.
One problem with this technique is that the measurement will be affected by "lift-off", i.e. the separation between the coil and the coin, which is difficult to control accurately. To compensate for this effect, it is known to use a second coil on the opposite side of the coin, the two coils being connected together in the oscillator circuit. Thus, increased lift-off will diminish the effect of the coin on one coil, but increase the effect on the other coil.
Although this improves matters, it is still not possible to obtain a very high resolution measurement using this technique. This is primarily due to coin embossing, which effectively superimposes a noise on the measurement. The result is that the embossing can cause an effect on the diameter measurement which depends upon the orientation of the coin at the point at which the diameter measurement is taken (i.e. at the peak of the frequency shift caused as the coin passes the coils). Normally, the separation between the coils is quite large (to allow for different-thickness coins), the coin passing in close proximity to one of the coils, and being spaced further away from the other coil. In this situation the diameter measurement may also be dependent upon which face of the coin is closest to the nearest coil.
A different technique, which avoids the effects of embossing, involves again using two coils, but in this case one of the coils is driven to form a transmission coil, and the other is a receiving coil. As a coin passes between the coils, it effectively acts as a shield and the coupling, i.e. the mutual inductance, between the coils decreases. The degree to which this happens is a function of the coin diameter.
However, it is necessary for the transmission sensor to be driven at a very high frequency not merely to avoid the effects of coin thickness, but also to ensure that the coin acts as an effective shield. If the coin diameter increases, the received signal level decreases, so that it is necessary to sense low level signals at high frequencies, which is in practice difficult to achieve.